A projection type liquid crystal display device has features that are more advantageous than those of a projection type CRT (cathode-ray tube) display device. For example, a color reproduction range of the liquid crystal display device is wider than that of the CRT display device. Further, a small size and a light weight of the liquid crystal display device makes it easier to carry the liquid crystal display device. Moreover, since the liquid crystal display device is not affected by earth magnetism, a convergence adjustment is not necessary. Since, it is also easy to realize a large-size screen, the liquid crystal display device is expected to be a mainstream of home-use image display device.
Among color projection type image display modes utilizing liquid crystal display element, there is three-panel mode in which three panels of liquid crystal display element are respectively used for three primary colors, and a single-panel mode in which only one panel of liquid crystal display element is used. In the three-panel mode, an optical system and three panels of liquid crystal display element are independently provided. The optical system divides a white beam into three primary colors, R, G and B, and the respective color beams are controlled by the three panels of liquid crystal display element, so as to form an image. Thus, full color display is achieved by optically superimposing images of the respective colors.
Some of the advantages of the three-panel mode are that light being emitted from a white beam source is efficiently utilized, and that the color purity is high. However, it is difficult to reduce the cost and size of a liquid crystal display device of the three-panel mode, as it requires the color separation function and the image formation function, which consequently increases the number of components thus complicating the optical system of the liquid crystal display device.
On the other hand, the single-panel mode utilizes only one panel of liquid crystal display element with a color filter pattern shaped in mosaic, stripe, or the like for three primary colors. Images are displayed by projecting light on the liquid crystal display element, using an optical system for projection. The single-panel mode is suitable for low-cost and small projection systems, because it requires only one panel of liquid crystal display element, and the structure of its optical system is simpler than one in the three-panel mode.
However, a disadvantage of the single-panel mode is that it can use only a ⅓ of incident light due to absorption or reflection of light that occurs at its color filter. In order to solve this problem, a color-filterless device, in which two layers of micro-lens arrays are provided on a light-source facing side of the liquid crystal display element has been introduced.
In such a device, dichroic mirrors are arranged in a sector form. These dichroic mirrors divide white beam from a white beam source into R, G and B, and cause the divided beams to enter the first micro-lens array at different angles. After passing the first micro-lens array, the light beams of the respective colors are refracted by a second micro-lens array so that the principal rays of the respective colors R, G and B separated by the dichroic mirrors become substantially parallel to one another. The respective light beams refracted by the second micro-lens array separately fall on liquid crystal regions driven by signal electrodes that are independently impressed with color signals corresponding with R, G and B (e.g. see Japanese Unexamined Patent Application No. 181487/1995 (Tokukaihei 7-181487; published on Jul. 21, 1995)).
The device does not use an absorbing color filter; therefore, not only the efficiency of using light improves but also the principal rays of respective colors after passing the micro-lens arrays are made substantially parallel to one another. As a result, it is possible to provide remarkably bright images by restraining diffusion of the principal rays of the respective colors before they reach a projection lens, and by preventing decrease in light quantity caused by vignetting.
Nevertheless, the foregoing color-filterless projection type liquid crystal display device of the single-panel mode, in which the efficiency of using light is improved, causes a relatively higher cost amongst the devices of the single-panel mode.
This is attributed to difficulties in manufacturing the second micro-lens array, which is one of two micro-lens arrays, for causing the principal rays of plural light beams to be parallel to each other. Amongst the plurality of beams incident to a lens of the second micro-lens array, a light beam incident to an edge portion of a lens of the second micro-lens array is deflected by the second micro-lens array so as to be parallel to the principal ray of a light beam incident to a center of the lens. This requires the lens (the second micro-lens) to have a large refraction power.
Conventionally, the second micro-lens array has lenses having a spherical surface or a cylindrical surface. The lens having the spherical surface, and the lens having the cylindrical surface both requires more thickness in order to obtain the large refraction power. This causes manufacturing of the lens to be extremely difficult, and causes an increase in the cost.
Further, a trapezoidal lens is sometimes adopted for the second micro-lens array (e.g., see Japanese Unexamined Patent Application 50081/1997 (Tokukaihei 9-50081; Published on Feb. 18, 1997)). It is easy to increase a thickness of the trapezoidal lens, for increasing the refraction power thereof. However, due to a converged light beam incident to a slanted surface of the trapezoidal lens, a refractive angle at inside of the light beam and that at outside of the light beam differ from each other. This causes a coma aberration and an astigmatism. The occurrence of these aberrations are not preferable, because they deteriorate the efficiency of using light.
The present invention was made in view of the foregoing problems, and it is an object of the present invention to realize a single-panel mode color-filterless projection type liquid crystal display device with high light-using efficiency at a low cost, without deterioration in light-using efficiency.